Parade

Ask Marilyn: Why Can I See My Breath Sometimes But Not Others?

Joe Heard of Denver writes:

Marilyn: When it’s cold outside, sometimes you can see your breath; other times, you ?can’t. What determines this? Marilyn responds:

It depends on the humidity. When the water vapor in your breath exceeds the amount that can be held by the air, the vapor condenses into tiny droplets that form the ­familiar short-lived little cloud. The sight doesn’t last long because the droplets evaporate quickly in the dry outdoor air, which is always in motion.

Actually, the droplets are so small that you can’t see them, but they’re made ­visible by the light they ­reflect. That’s why you can see your breath much better on a sunny day. And you may be surprised to hear that the coldest temperatures are ­often recorded on bright, sunny days ­because there’s no blanket of clouds to keep heat on the ground.

You can see your breath only in winter because the colder the air, the less moisture it can hold (warm air can ­always hold more water vapor than your breath can). So very cold, dry, clear days mean the conditions are just right for playing dragon with the kids.